Add temporary faux api to allow popups to constrain popup placement
We want popups to not overlap other panels. We do not know these locations from within libplasma window code.
Short term, this patch adds a property to allow client code to set the availableScreenGeometry that should be used to constrain the window.
Mid-term will be addressed properly in Plasma 6.2 with a singleton that tracks available screen geometry which is described at #22 (comment 943229) with pending MRs, but it's too late for 6.1
Long-term (on wayland) kwin will position the window semantically itself and this comes for free.
The code is not the prettiest, but there is a plan scheduled and it's a good test of the underlying workflow we want without committing to API.